Oxygen enhanced combustion of biomass

ABSTRACT

The energy output of a power plant combustion chamber that combusts fuel comprising biomass as all or part of the fuel can be increased by feeding oxygen into the combustion chamber so that said fuel is in contact with gaseous oxidant whose oxygen content exceeds that of air by up to 5 vol. % above that of air.

RELATED APPLICATION

This application is a continuation application of and claims priorityfrom U.S. patent application Ser. Number 13/285,654 filed Oct. 31, 2011,which claims priority from U.S. Provisional Application Ser. No.61/412,119 filed Nov. 10, 2010.

FIELD OF THE INVENTION

The present invention relates to combustion of biomass, especially inpower plants that generate steam.

BACKGROUND OF THE INVENTION

Growing demand for electrical power obtained from fuel-fired powerplants, combined with growing interest in using biomass as fuel for suchplants, has increased interest in finding efficient methods forcombusting biomass in power plants. The moisture content of biomass istypically very high. For example green wood typically contains 40 to 60%moisture. This increased moisture content, and its low energy density,are among the primary issues with firing biomass in boilers andespecially boilers that were designed for other fuels such as coal. Forexample, converting a coal-fired boiler to fire biomass typically causethe boiler to be derated by 30-50%.

Many boilers are ‘flue gas limited’ and can only handle up to a specificamount of flue gas. This flue gas limitation may be due to the capacityof fans if present for impelling flow of flue gas, or may be based ondesign limits. For example, boiler design considerations, such as themaximum allowable velocity in the convective section, can limit flue gasvolume. Since the flue gas volume per unit heat input, or “specific fluegas volume”, increases dramatically when a fuel such as coal is replacedwith biomass, it causes a large impact on the distribution of heatabsorption in the furnace. A boiler is typically designed for arelatively narrow range of specific flue gas volume. Within this rangethe boiler is designed for a specific amount of heat absorption in thefurnace, or radiant section, and the convective section. When thespecific flue gas volume is increased more heat is ‘pushed’ from theradiant section into the convective section. This increase in heattransfer in the convective section often requires the use of watersprays into the steam flow to maintain the desired steam temperature,which may decrease overall efficiency. This shifting of heat transferfrom the radiative furnace section to the convective section of theboiler further reduces or derates the boiler capacity.

Conversion of an existing boiler to biomass firing can alsosignificantly degrade the combustion performance of the unit. Thereduction in combustion performance is due to both changes in the fuelcharacteristics and the firing system. The high moisture content thefuel makes it more difficult to ignite and burn. This problem iscompounded by the fact that grate firing systems often suffer fromuneven fuel distribution over the grate and non-uniform mixing betweenthe air and the fuel—leading to incomplete combustion on parts of thegrate and high CO emissions in flue gas. To overcome both of theseproblems boiler operators typically operate the boiler at increasedstoichiometric ratios (defined as the ratio of air supplied to thatrequired to burn the fuel). The stoichiometric ratio is often measuredas the amount of oxygen left in the flue gas at the end of thecombustion process. For example, a typical coal-fired boiler willoperate with 3% “excess oxygen”. This means the flue gas contains 3%oxygen (by volume, wet basis). In contrast the flue gas from abiomass-fired boiler typically contains at least 4.5% O₂ (vol, wetbasis) to control CO emissions within regulatory limits.

The extra air further increases the flue gas volume and impacts both thethermal efficiency of the boiler, and the auxiliary power required forthe boiler. In the first case the extra air volume carries heat out thestack, increasing the sensible heat loss. The extra air also increasesthe power required by both the blower that pushes combustion air intothe boiler (typically called the forced draft, or FD, fan), and theblower used to draw the flue gas from the boiler (typically called theinduced draft, or ID, fan). Therefore the overall effect of the excessair is to increase the specific flue gas volume, which is the gas volumeper units of energy output (further limiting the amount of fuel that canbe fired), reduce the thermal efficiency (allowing less of the fuel thatis fired to be used to raise steam), and increase the auxiliary power(reducing the net power available

The present invention provides an improved method for combustion ofbiomass in boilers.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of combustion,comprising

(A) providing apparatus that includes a combustion chamber in which fuelhaving a given moisture content and a given specific energy content fedinto the combustion chamber at a given mass feed rate can be combustedin air to produce heat energy at a given rate,

(B) feeding into said combustion chamber fuel that contains biomass andthat has a specific energy content lower than said given specific energycontent, so that combustion in air of said fuel fed at said given massfed rate in said combustion chamber in air produces heat energy at arate lower than said given rate, while feeding oxygen into saidcombustion chamber so that said fuel is in contact with gaseous oxidantwhose oxygen content exceeds that of air by up to 5 vol. % above that ofair, and

(C) combusting the fuel with said gaseous oxidant in said combustionchamber.

Another aspect of the invention is a method of increasing fuelcombustion rate in a combustion chamber with a convective heat transferzone in which fuel that contains biomass is combusted with combustionair in said combustion chamber to produce flue gas containing a specificoxygen concentration between 3 vol. % and 8 vol. % at a given maximumfuel feed rate limited by the capacity of an FD fan if present forfeeding said combustion air, the capacity of an ID fan if present toevacuate flue gas from said combustion chamber, the flue gas velocity insaid convective heat transfer zone, or the carbon monoxide concentrationin said flue gas, feeding into said combustion chamber additional fuelcontaining biomass and additional oxidant containing at least 50 vol. %O₂, reducing said combustion air flow rate by the amount that reducessaid oxygen concentration in said flue gas by 0.1 to 5.0 vol. % andcombusting said additional fuel without exceeding said FD fan capacity,said ID fan capacity, said flue gas velocity, nor said carbon monoxideconcentration.

Yet another aspect of the invention is a method of increasing fuelcombustion rate in a combustion chamber with a grate for combustion offuel with a convective heat transfer zone in which fuel that containsbiomass is combusted with combustion air in said combustion chamber toproduce flue gas containing a specific oxygen concentration between 3vol. % and 8 vol. % at a given maximum fuel feed rate limited by thecarbon monoxide concentration in said flue gas, feeding into saidcombustion chamber additional fuel containing biomass and additionaloxidant containing at least 50 vol. % O₂ to one or more oxygen deficientareas on said grate to maintain or reduce said carbon monoxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of one embodiment of combustionapparatus in which the present invention can be practiced.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improvement in the combustion of fuelcomprising biomass in a combustion chamber. “Biomass,” for the purposesof the present invention, means any material not derived from fossilresources and comprising at least carbon, hydrogen, and oxygen. Biomassincludes, for example, plant and plant-derived material, vegetation,agricultural waste, forestry waste, wood, wood waste, paper waste,animal-derived waste, poultry-derived waste, and municipal solid waste.Other exemplary feedstocks include cellulose, hydrocarbons,carbohydrates or derivates thereof, and charcoal. Typically biomass caninclude one or more materials selected from: timber harvesting residues,softwood chips, hardwood chips, tree branches, tree stumps, leaves,bark, sawdust, off-spec paper pulp, corn, corn stover, wheat straw, ricestraw, sugarcane bagasse, switchgrass, miscanthus, animal manure,municipal garbage, municipal sewage, commercial waste, grape pumice,almond shells, pecan shells, coconut shells, coffee grounds, grasspellets, hay pellets, wood pellets, cardboard, paper, plastic, andcloth. The present invention can also be used for fuels that alsocomprise carbon-containing feedstocks other than biomass, such as afossil fuel (e.g., coal or petroleum coke), i.e. mixtures of biomass andfossil fuels.

The present invention is especially applicable to combustion of biomassin a combustion chamber that is part of a system that includes, inaddition to a combustion chamber, heat exchangers that absorb heat ofcombustion into, for instance, water. Preferred systems include powergeneration boilers, especially in which heat exchange to boiler feedwater is achieved by radiant heat transfer and by convective heattransfer. The heat exchange produces steam, superheated steam, and/orsupercritical steam, which can be used to generate electric power.

The present invention is especially applicable to combustion of biomassin a combustion chamber including a grate on which fuel rests as it isbeing combusted. However, the present invention can be practiced insystems wherein the fuel is combusted in the combustion chamber by gratefiring, suspension firing, or a combination of grate firing andsuspension firing, or by firing in a bubbling fluidized bed or in acirculating fluidized bed.

The following description refers to FIG. 1, and illustrates practice ofthe invention in one embodiment in which grate firing is employed.

Combustion chamber 1 includes grate 2 on which fuel can rest after thefuel is fed into combustion chamber 1, for instance as fuel stream 3.Grate 2 is solid and includes a plurality of openings through which gascan flow, including primary air which is fed as primary air stream 4.Optionally, overfire air stream 5 can also be fed into the combustionchamber 1.

Combustion of fuel in combustion chamber 1 produces heat of combustion,and flue gas which exits combustion chamber 1 as stream 7. The heat ofcombustion can be transferred to feed water flowing through boiler tubesin the walls of combustion chamber 1, to heat the feed water. Heat ofcombustion can also be transferred from flue gas by indirect heatexchange to feed water, or to steam, in heat section 6 which generallyincludes a region (the “radiant section”) in which heat transfer occurspredominantly by radiative heat transfer, and a region (the “convectivesection”) in which heat transfer occurs predominantly by convective heattransfer.

In the present invention, oxygen is fed in small amounts into the regionbelow grate 2, into the region of the fuel on grate 2, or into bothregions. Sufficient oxygen is fed so that the gaseous atmosphere incontact with the fuel has an oxygen content higher than that of air,i.e. at least 21 vol. %, up to 5 vol. % higher than that of air andpreferably not more than 1 vol. % higher than that of air.

The oxygen can be fed into the region below grate 2 in any of numerousways, such as by mixing it with primary air that is fed as stream 4, orinserting a lance 8 into the region below grate 2 and feeding the oxygenthrough the lance into the region below grate 2 where it then can mixwith primary air.

The oxygen can be fed into the region above grate 2 in any of numerousways, such as by inserting a lance 9 into the region above grate 2 sothat oxygen emerging from the lance 9 can contact fuel present on thegrate, and feeding oxygen through the lance 9.

The oxygen that is fed below or above the grate 2 is preferably fed as astream comprising at least 50 vol. % oxygen preferably 90 vol. % oxygen.Streams having such oxygen content are readily available from commercialsources. Alternatively, streams having such oxygen content can be formedin apparatus located near the combustion chamber such as VPSA units thatseparate oxygen from air.

The practice of the present invention provides numerous advantages inits own right, and especially compared to prior practice relating tocombustion of biomass.

The moisture content of fuel comprising biomass is typically very high.This increased moisture content, and its low energy density, are amongthe primary issues with firing biomass in boilers and especially boilersthat were designed for other fuels. For example, converting a 50MW_(net)coal-fired boiler (heat rate of 11,500 Btu/kWh_(net)) to fire biomasswould be expected to cause the boiler to be derated by 20-45% just toaccount for the moisture in the fuel. The shift in boiler heat transferbalance and the increased excess air requirement increase the requiredderate to 30-50% for many boilers. The present invention permitsefficient combustion of biomass fuels, even in boilers that weredesigned for combustion of fuels having lower water contents, and/orhigher energy density, than biomass. The invention is useful when thefuel containing the biomass has a water content of at least 25 wt. %, orwhen the fuel containing the biomass has an energy content less than7500 BTU/lb or even less than 5000 BTU/lb.

In the present invention, the addition of only a small amount of oxygenenhances and controls combustion both on and above the grate as a meansto recover lost generating capacity. The enhanced combustion, in turn,enhances flame stability and ensures more complete burnout. Oxygeninjection over the grate can also stabilize and improve the combustionprocess. In general, by using oxygen in the combustion environmentaccording to the present invention, it is possible to reduce the excessair flow, and thereby reduce the specific flue gas volume. The lowerspecific flue gas volume allows the boiler operator to increase thefiring rate to regain some of the generating capacity lost when theboiler was converted to biomass firing. Even small reductions in excessair can allow boiler capacity lost during the conversion to biomass tobe recovered (reducing the required boiler derate).

Another operational benefit of oxygen injection according to the presentinvention is that less heat will be ‘pushed’ into the convective sectiondue to both the reduced specific flue gas volume and the increasedtemperature near the fuel bed on the grate. Both of these effects leadto increased heat absorption in the radiative part of theboiler—reducing the need to spray in cooling water to control superheatand reheat temperatures in the convective section.

In the present invention oxygen could be added by combination of beingdirectly injected or mixed with combustion air (enrichment). Forexample, one might enrich the undergrate air to ensure there are no “hotspots” nor “cold spots” on the grate, while using high momentum lancesto inject oxygen above the grate to promote good mixing and volatiles/COburnout. The over-bed oxygen lances can also be used to move heat (byinfluencing mixing) into different parts of the grate. For example, someof the heat from the volatile combustion zone of the grate can be movedinto the drying portion of the grate to facilitate drying. Overfire air5 (air supplied through ports located at one or more elevation from thegrate) can also be enriched to enhance volatile combustion.Alternatively oxygen enrichment under the grate may be increased throughthe use of a lance to target areas where the grate is known to be‘cold’, or combustion is poor. The amount of oxygen required to recovercapacity by enabling reduced excess oxygen operation is much less thanthat estimated for a simple direct replacement of combustion air. Forexample, the stoichiometric oxygen requirement for a typical dryash-free wood is about 2,000 SCF (123 lb) per 1,000,000 Btu and producesabout 3,200 SCF of flue gas. Conversely, 1 lb of oxygen can combustabout 8130 Btu of fuel and produces 26 SCF of flue gas. In order tomaintain the original flue gas volume and burn additional fuel a portionof the original combustion air volume must be reduced and replaced withadditional oxygen. The oxygen requirement to increase the capacity (orfuel firing rate) by 10% under the condition of constant flue gas volumeflow rate was calculated for both dry and wet wood with 45% moisturecontent at two different excess oxygen levels (3 and 4.5% by volume inwet flue gas) and summarized in Table 1. The amount of oxygen requiredranges from 2850 to 3410 SCF per MMBtu of additional fuel input at theconstant excess O₂ in flue gas. By reducing the excess oxygen level by 1vol. %, the amount of oxygen required is reduced to less than half, in arange from 1140 to 1510 SCF per MMBtu of additional fuel input.

TABLE 1 Oxygen required (SCF/MMBtu): Constant 1% reduction Biomassexcess O2 in excess O2 Dry wood, 3% Excess O2 2850 1260 Dry wood, 4.5%Excess O2 2870 1140 Wet wood, 3% Excess O2 3410 1510 Wet wood, 4.5%Excess O2 3410 1330

The current invention has several additional advantages. First, by usingonly enough oxygen enrichment to achieve flame stability on and abovethe grate, gross changes to furnace operation can be avoided. Forexample, many furnaces are designed for a specific heat absorptionpattern. In a steam boiler for power generation the balance between heattransfer in the radiant (furnace) section is often carefully balancedwith that in the convective section by the boiler designer. Variationsin heat transfer pattern from the design point can cause significantupsets in boiler operation. When high oxygen enrichment levels, such asthose presented in the prior art (>25%) are used, the heat transfer tothe radiant section is often dramatically increased. For a utilityboiler this means the steaming rate (rate of steam production) isincreased, but there is insufficient heat available to superheat thesteam to the desired turbine inlet temperature. In the current inventionthe transition to a high moisture fuel often leads to off-design furnaceoperation where heat transfer to the radiant section is reduced comparedto the design case. By using a small amount of oxygen enrichment andthereby reducing the excess air requirement the radiative/convectiveheat transfer balance can be restored, at least in part, withoutincreasing the radiative heat transfer past the design limits.

The present invention also does not require exhaust gas recirculationfor over-grate mixing. This leads to a much lower capital requirement(EGR fans, ducts, and the like) and reduced operating cost.

Additionally, by using the oxygen addition of the present invention onlyto support combustion and thereby reduce the specific flue gas volumethrough excess air reduction, the volume reduction compared to oxygenuse is much higher than in the prior art. This enhanced effectiveness ofoxygen addition for flue gas reduction leads to much lower oxygenrequirements.

A significant advantage of the current invention over the prior art isrelated to the use of oxygen enrichment only to support combustion andthereby reduce the specific flue gas volume through excess airreduction, the flue gas volume reduction compared to the simplereplacement of a portion of combustion air with oxygen is much higherthan in the prior art. This enhanced effectiveness of oxygen additionfor flue gas reduction leads to much lower oxygen requirements. Anexample for converting a 20 MW_(net) coal-fired boiler to fire biomassis shown in Table 2. For these calculations the flue gas volume was heldconstant, consistent with a flue gas limited boiler. The baselinegenerating capacity was defined as that after the boiler was convertedto biomass firing (using a 32% moisture fuel) and was 14.7 MWnet in thisexample. The increased generating capacity was first estimated assumingthe oxygen concentration in the flue gas was held constant at 4.5% (vol,wet) and combustion air was replaced with increasing levels of oxygen.This condition is the conventional ‘volume reduction’ strategy where thenitrogen in the combustion air is simply removed by using oxygen inplace of a portion of the air. As can be seen in Table 2, the generatingcapacity can be increased significantly, but the oxygen requirements arehigh enough that oxygen use may not be economically justified. In thecase of the current invention, kinetic data was used to estimate theincrease in firing rate from oxygen enrichment. The air injection ratewas reduced by the amount of oxygen injected and the firing rateincreased—resulting in a lower oxygen concentration in the flue. Withinjection targeted to particular locations in the combustion chamber,such as described below, the oxygen consumption may be even lower. Thedata in Table 2 show that the oxygen use is dramatically lower for agiven increase in capacity for the current invention. Using oxygen inthis way can be economically viable.

TABLE 2 Increase in generation Oxygen required (SCF/MW baseline): (% ofbaseline) Volume reduction Present invention 2% 710 70 10% 3500 340 22%7420 1020“Volume reduction” means operating such that the reduction in specificflue gas volume is attained only by the replacement of air with an equalamount of oxygen.“Present invention” means operating such that the reduction in specificflue gas volume is attained in part by reduction in the amount of excessair.The optimal embodiment of the current invention uses small amounts ofoxygen to support the various stages of biomass combustion. These stagesinclude:

-   -   Preheating/drying,    -   Volatile release,    -   Volatile combustion,    -   Char combustion.

In a grate fired-combustor, such as that shown in FIG. 1, these stepscan occur in-flight or on the grate, depending on the fuelcharacteristics (size) and fuel spreader/boiler design. For example,fine particulate are likely suspended as they are ‘thrown’ into thefurnace. Therefore for the fine materials the entire combustion processoccurs in flight. For the largest particles they may dry slightly asthey exit the fuel spreader but land on the grate before drying iscomplete. Therefore, for these particles the combustion process occursprimarily on the grate. Combustion problems can occur when the fuel andair distribution are not matched across the grate and overfire air. Forexample, if too much fuel is deposited on a specific portion of thegrate the combustion air may be insufficient to burn the material.Although optimal overfire air designs promote good mixing above thegrate, there may still be regions where the oxygen levels are too low tocomplete combustion (and other areas where the excess air is much higherthan required for combustion). Further, the heat release pattern fromthe volatile combustion may not match that required to promotedrying/devolatilization of materials that have landed on thegrate—causing material on portions of the grate to ‘smolder’ instead ofburn.

It is known that high levels of oxygen enrichment can enhance combustionand overcome problems associated with air/fuel distribution and heatrelease mismatches. However, the objective of the current invention isto use the least amount of oxygen to enable the excess air to be reduced(and thereby enable an increase in boiler firing rate). Therefore theoptimal embodiment is to use a lance, or lances, above the grate toinject oxygen into oxygen-deficient areas above the grate. Often theoxygen deficient area looks darker than the rest of the grate as thelocal temperature is colder. Such area can be detected by in-furnacevideo camera, by an optical pyrometer or by visual observation. Othermethods of detecting the oxygen deficient area include gas analysisusing a gas sampling probe and by an optical gas species measurementdevice. With careful lance design mixing can be controlled between theinjected oxygen and the oxygen deficient (and likely high CO) flue gas.Further, by targeting the injected oxygen jet tragectory high oxygencontaining flue gas ‘pockets’ in the furnace atmosphere can be drawninto the oxygen deficient area. The combination of aerodynamic effectsfrom the lance design and the kinetic effect of high oxygenconcentrations enhance volatiles and CO combustion. The over-gratelances can also be used to ‘move’ volatile combustion to add heat tocooler portions of the grate to support the combustion process on thegrate.

In addition to the over-grate lances the optimal embodiment can also usedirected oxygen enrichment under the grate to enhance combustion onspecific regions of the grate. For example, if the windbox under thegrate has partitions to divide the airflow to different parts of thegrate, different levels of oxygen enrichment could be used in thedifferent partitioned areas (through use of oxygen distributors in theair supply duct for each partition). Alternately a carefully designedoxygen injection lance could be installed either below the grate orimmediately above the grate to enrich the combustion air in theimmediate vicinity of a known ‘cold spot’, or oxygen deficient areas.

What is claimed is:
 1. A method of combustion, comprising (A) providingapparatus that includes a combustion chamber in which fuel having agiven moisture content and a given specific energy content fed into thecombustion chamber at a given mass feed rate can be combusted in air toproduce heat energy at a given rate, (B) feeding into said combustionchamber fuel that contains biomass and that has a specific energycontent lower than said given specific energy content, so thatcombustion in air of said fuel fed at said given mass fed rate in saidcombustion chamber in air produces heat energy at a rate lower than saidgiven rate, while feeding oxygen into said combustion chamber so thatsaid fuel is in contact with gaseous oxidant whose oxygen contentexceeds that of air by up to 5 vol. % above that of air, and (C)combusting the fuel with said gaseous oxidant in said combustionchamber.
 2. The method of claim 1 wherein the fuel has an energy contentof less than 7500 BTU/lb.
 3. The method of claim 1 wherein in step (B)oxygen is fed into said combustion chamber so that said fuel is incontact with gaseous oxidant whose oxygen content exceeds that of air byup to 1 vol. % above that of air.
 4. A method of increasing fuelcombustion rate in a combustion chamber with a convective heat transferzone in which fuel that contains biomass is combusted with combustionair in said combustion chamber to produce flue gas containing a specificoxygen concentration between 3 vol. % and 8 vol. % at a given maximumfuel feed rate limited by the capacity of an FD fan if present forfeeding said combustion air, the capacity of an ID fan if present toevacuate flue gas from said combustion chamber, the flue gas velocity insaid convective heat transfer zone, or the carbon monoxide concentrationin said flue gas, feeding into said combustion chamber additional fuelcontaining biomass and additional oxidant containing at least 50 vol. %O₂, reducing said combustion air flow rate by the amount that reducessaid oxygen concentration in said flue gas by 0.1 to 5.0 vol. % andcombusting said additional fuel without exceeding said FD fan capacity,said ID fan capacity, said flue gas velocity, nor said carbon monoxideconcentration.
 5. The method of claim 4 wherein said oxygenconcentration in said flue gas is reduced by 0.1 to 1.0 vol. %.
 6. Themethod of claim 4 wherein said additional oxidant contains at least 90%vol. O₂.
 7. The method of claim 4 wherein the ratio of the oxygencontained in said additional oxidant to said additional fuel is lessthan 2,000 SCF/MMBtu.
 8. The method of claim 4 wherein the ratio of theoxygen contained in said additional oxidant to said additional fuel isless than 1,500 SCF/MMBtu.
 9. The method of claim 4 wherein said fuelcombustion rate in said combustion chamber is increased by 3% to 30% inBtu content.
 10. The method of claim 4 wherein said additional oxidantis injected to one or more oxygen deficient areas in said combustionchamber.
 11. The method of claim 4 wherein said additional oxidantcontains at least 90% vol. O2.
 12. A method of increasing fuelcombustion rate in a combustion chamber with a grate for combustion offuel with a convective heat transfer zone in which fuel that containsbiomass is combusted with combustion air in said combustion chamber toproduce flue gas containing a specific oxygen concentration between 3vol. % and 8 vol. % at a given maximum fuel feed rate limited by thecarbon monoxide concentration in said flue gas, feeding into saidcombustion chamber additional fuel containing biomass and additionaloxidant containing at least 50 vol. % O₂ to one or more oxygen deficientareas on said grate to maintain or reduce said carbon monoxideconcentration.
 13. The method of claim 12 wherein said oxygenconcentration in said flue gas is reduced by 0.2 to 1.0 vol. %.
 14. Themethod of claim 12 wherein said additional oxidant contains at least 90%vol. O2.
 15. The method of claim 12 wherein said additional oxidant isinjected from above said grate, from below said grate, or from bothabove and below said grate.